An autoinjector is a drug delivery device which contains a medical, therapeutic, diagnostic, pharmaceutical or cosmetic compound (drug) before it is administered, and which is used to administer the compound through the skin of the patient via a hollow needle. Autoinjectors may be used by the patient themselves or by a different user, and are also used to administer drugs to animals.
Autoinjectors are typically used because they reduce the amount of training and effort needed by a user compared with that needed for a syringe, by automating either or both processes of inserting the needle into the patient and expelling the drug through the needle. They can also reduce the fear of injection by hiding the needle from the patient
Autoinjectors typically include a housing containing a drug and a plunger that is driven by an automatic mechanism to move the plunger within the housing to eject the drug. The automatic mechanism may also move the needle relative to the housing to insert the needle into a subject. Motive power for the mechanism may come from one or more springs or other power sources such as compressed gas.
Autoinjectors are used to deliver so-called crisis drugs such as epinephrine, where a patient may need to self-inject the drug while under the severe stress of anaphylactic shock. They are also used to deliver drugs for long-term conditions such as rheumatoid arthritis, where the patient may have limited dexterity. In both cases it is beneficial for the autoinjector to have a simple and easy user interface in order to maximise the likelihood that the patient is able to operate the autoinjector correctly and receive the drug. Some autoinjectors include a finger-operated button or other control to allow the patient to activate them, but this approach can be confusing and more difficult to use. Other autoinjectors advantageously incorporate a very simple user interface design where the autoinjector is activated and the drug delivered by the action of the patient pressing a skin sensor component against the injection site.
Autoinjectors typically have a housing which encloses a needle at the front end close to the injection site, a drug container, and one or more drive members such as springs towards the back of the device at the other end from the injection site. Typically an autoinjector is manufactured as partial subassemblies by a specialist device manufacturer, and these partial subassemblies are then assembled with a filled drug container at another site, often the filling facility for the drug container, to form the complete autoinjector. The device manufacturers are generally not able to handle the drug component. Equally, the filling facilities generally lack device assembly expertise or capability. For this reason it is advantageous to be able to safely transfer autoinjector subassemblies from the device manufacturer to the final assembly facility, and then assemble them with the filled drug container with minimum assembly operations and manufacturing complexity.
In order to allow the drug container to be assembled into the final autoinjector, autoinjectors are typically shipped to the final assembly location as two subassemblies, a front subassembly including a skin sensor if incorporated into the design, and a rear subassembly including the power source in an energised condition, and a button if incorporated into the design. This requires the rear subassembly to contain the stored energy source safely before final assembly, without releasing the energy prematurely.
Autoinjectors with activation buttons positioned on the rear subassembly of the autoinjector typically rely on this button to retain the stored energy source safely before final assembly. However if the design of the autoinjector is such that the autoinjector is activated only by the action of the patient pressing a skin sensor at the front end of the autoinjector onto the injection site, without the use of a separate finger-operated button, then there is a requirement for the rear subassembly to contain the stored energy source safely before final assembly, but to allow it to be released when needed during activation of the skin sensor which is in the front subassembly. This can be difficult and complex to achieve, and can increase the size and cost of the autoinjector. There is a risk that energy source can be released before final assembly due to handling forces.
It is an object of the present invention to provide a drive mechanism for an autoinjector that can be safely shipped in locked state, in which it cannot be activated, but which can be simply assembled to another component or components to form an autoinjector, in which the drive mechanism can be simply activated.